This invention relates to the field of phased array antennas that are controlled by networks of optical fibers and other photonic components, such as photonic beamforming networks. More specifically, it relates to methods for constructing low-loss, passive photonic beamforming networks.
Phased array antenna systems are widely used in radar, electronic warfare and high data-rate communications applications. They are sometimes controlled by networks of optical fibers and other photonic components such as lasers, fiber splitters/combiners, and photodetectors. These control networks mainly utilize delay line networks such as the ones shown in FIG. 1. There are two types of delay line networks which differ in the way time delays are implemented. In the network switched architecture of FIG. 1a, of which the Rotman lens is an example, entire networks of delay lines are switched in/out by a single switch. In the in-line switched architecture of FIG. 1b, there are several delay lines within each fiber as well as a switch to select them. If F is the number of fibers and P the number of delay states, the network switched architecture requires one switch with P states, and the in-line switched architecture F switches with P states. Both require 1xc3x97P splitters to access P delay states, and Fxc3x971 combiners to vector sum the outputs. For both types of networks, the signal passes through one 1xc3x97P splitter, one switch, and one Fxc3x971 combiner, so the losses are expected to be comparable.
The number of photodetectors required in the network can be a major cost driver so it is desirable to minimize it. To achieve this, one can place a single photodetector at position A in FIGS. 1a and 1b, after the Fxc3x971 combiner which vector sums the fiber signals. However, if all fibers carry the same optical wavelength, as it is the case in most prior art systems, the different signals will interfere and unwanted noise will appear on the detected carrier envelope. In order to avoid this optical coherence problem, photodetectors can be placed at positions B so that photodetection occurs prior to summation, and the optical carriers never interact. However, a large number of photodetectors is then required and cost is greatly increased.
To solve optical coherence problems, while still minimizing the number of photodetectors required, this invention utilizes multiple optical wavelengths. This reduces photodetector count from Fxc3x97P to 1 in the network switched case (FIG. 1a), and from F to 1, in the in-line switched case (FIG. 1b).
Furthermore, in accordance with this invention, the lossy splitters/ combiners that form the actively switched prior art networks of FIGS. 1a and 1b, are replaced by a passive Wavelength Division Multiplexing (WDM) network. The 1xc3x97P splitters and Fxc3x971 combiners are replaced with WDMs, and the functions performed by active switches are realized by separating wavelength groups with passive WDMs. Optical losses in a 1xc3x97N WDM are less than in a 1xc3x97N splitter or combiner for N greater than 6, so in most practical cases losses can be substantially reduced.
Prior art photonic networks require active switching, the use of a large number of photodetectors, and inclusion within the network of lossy splitters and combiners. In many cases the prior art also requires specialized or unique optical components.
Prior photonic beamforming art such as described in U.S. Pat. No. 5,861,845 (Wideband Phased Array Antennas and Methods) alludes to using multiple wavelengths to avoid optical coherence effects, but losses are still high in the combiners which vector sum the optical signals. patent application Ser. No. 09/383,819 (Phased Array Antenna Beamformer) describes a passive receiver network for multiple beams which employs WDMs for beam scanning and delay line selection.
However, it does not address optical coherence problems, and uses three-dimensional fiber optics based delay line networks (fiber Rotman lens) which are hard to fabricate. It also utilizes lossy combiners for signal summation.
The present invention addresses and solves these problems in a simple, unified manner, and can be implemented using standard ITU (International Telecommunication Union) components developed commercially for fiber optics data networks, and two-dimensional SOS (Silicon on Sapphire) fabrication techniques.
In accordance with the present invention, a number N of incoming RF wavefronts are simultaneously received by an antenna array. Laser light is amplitude modulated to provide B=N synthesized optical beams. The synthesized optical beams are mixed with the incoming electrical wavefronts by optical modulation. The resultant N optical wavefronts, all traveling through common waveguides, are each directed to a predetermined set of delay lines, and subsequently separated and channeled into N separate waveguides. The original incoming wavefronts carried by the synthesized optical beams are now differentiated and can be photodetected and analyzed separately.
This invention discloses novel ways to perform these functions utilizing photonic beamforming networks. It provides methods for constructing low-loss, completely passive, high performance photonic beamforming networks that can simultaneously control beam scanning and delay line selection for multiple beams. The invention comprises three main methods which include:
(1) laser wavelength hierarchies,
(2) arrangements of wavelength division multiplexing (WDM) components, and
(3) re-use of laser wavelengths.
Multiple laser wavelengths are arranged in groups and subgroups (wavelength hierarchies) in the wavelength domain. By switching between these wavelength groupings, the arrangements of WDM components proposed herein enable the beamforming network to direct the beam signals through the proper time delay lines, and to differentiate multiple beams. No switching occurs within the network itself, only at the controlling lasers, and the network is completely passive. Furthermore, signal routing, beam differentiation, and beam vector summation occur with minimal losses due to the arrangements and choice of WDM components and interconnections. The invention also minimizes the number of photodetectors required, and only one photodetector per beam is needed in its most powerful form.
The method of laser wavelength re-use permits significant reduction in the number of wavelengths required for the beamformer to function. This allows the wavelengths to be limited to the standard ones specified by the International Telecommunication Union, even with phased array antennas that contain a very large number of elements.
Another aspect of the invention, a non-passive, output-switched network, that minimizes the number of wavelengths required is also disclosed.